This invention relates to hydraulic pressure relief valves and circuits and, more particularly, to rate damped relief valves and circuits.
A relief valve is used in hydraulic circuits to limit the maximum pressure that can develop. However in hydraulic circuits utilizing relief valves, undesirable momentary high pressures above the set limit or overshoot can develop in response to transient conditions. Such a transient condition, for example, can occur when a lowering load is brought to an abrupt halt generating momentary pressure rate increases in the system ranging between approximately 120,000 to 3,000,000 psi per second. Such repeated overshoots tend to shorten the service life of affected system components.
The present invention is directed to a novel hydraulic pressure relief valve and circuit that will eliminate the overshoot under most conditions and limit the maximum rate of pressure increase in the system.
To this end, the invention comprises a hydraulic pressure relief circuit which includes a main valve which can be either a slide or poppet type of conventional design, a flow restricting bleed flow orifice to control the main valve, a pilot valve having a differential area piston, and a flow restricting damping orifice in conjunction with an accumulator volume to provide over-damping of the pilot valve. System pressure is limited by opening the main valve and allowing flow from the system line back to tank. The main valve is controlled as a bleed servo wherein the pilot valve meters the bleed flow from the bleed flow orifice to tank thereby controlling the opening of the main valve. Fluid flow through the damping orifice, in conjunction with the accumulator, limits the rate of pressure rise which acts on the pilot valve thereby limiting the maximum rate of pressure increase in the main system circuit.
A fuller understanding of the invention may be had from consideration of the following description and claims taken together with the accompanying drawings.